Reproducibility and diurnal variation of the directional sensitivity of the cerebral pressure-flow relationship in men and women.

The cerebral pressure-flow relationship has directional sensitivity, meaning the augmentation in cerebral blood flow is attenuated when mean arterial pressure (MAP) increases versus MAP decreases. We used repeated squat-stands (RSS) to quantify it using a novel metric. However, its within-day reproducibility and the impacts of diurnal variation and biological sex are unknown. Study aims were to evaluate this metric for: ) within-day reproducibility and diurnal variation in middle cerebral artery (MCA; ΔMCAv/ΔMAP) and posterior cerebral artery (PCA; ΔPCAv/ΔMAP) and ) sex differences. ΔMCAv/ΔMAP and ΔPCAv/ΔMAP were calculated at 7 timepoints (08:00-17:00) in 18 participants (8 women; 24 ± 3 yr) using the minimum-to-maximum MCAv or PCAv and MAP for each RSS at 0.05 Hz and 0.10 Hz. Relative metric values were also calculated (%MCAv/%MAP, %PCAv/%MAP). Intraclass correlation coefficient (ICC) evaluated reproducibility, which was good (0.75-0.90) to excellent (>0.90). Time-of-day impacted ΔMCAv/ΔMAP (0.05 Hz: = 0.002; 0.10 Hz: = 0.001), %MCAv/%MAP (0.05 Hz: = 0.035; 0.10 Hz: = 0.009), and ΔPCAv/ΔMAP (0.05 Hz: = 0.024), albeit with small/negligible effect sizes. MAP direction impacted both arteries' metric at 0.10 Hz (all < 0.024). Sex differences in the MCA only ( = 0.003) vanished when reported in relative terms. These findings demonstrate that this metric is reproducible throughout the day in the MCA and PCA and is not impacted by biological sex. The findings of the current study indicate that our time-adjusted metric to evaluate the directional sensitivity of the cerebral pressure-flow relationship is reproducible throughout the day in both the anterior and posterior cerebral circulations. Although a diurnal variation was noted across the day within this metric, this appeared to be of minimal physiological relevance. Finally, the metric is not impacted by biological sex.